1. Technical Field
The invention relates to brake system control of a motor vehicle, and more particularly to control of braking where a vehicle includes regenerative braking capability and conventional service brakes.
2. Description of the Problem
Various types of hybrid and electric vehicles obtain higher operating efficiencies and extend operating range by using regenerative braking. During regenerative braking a vehicle's kinetic energy is captured converted to a form amenable to storage. For example electrical energy may be stored in capacitors or subjected to conversion to potential chemical energy and stored in batteries or capacitors, or the energy may be stored mechanically by compressing a fluid. Later, the stored energy can be used to propel the vehicle. In the case of electrical power it can be applied as electricity to a traction motor, and on a hydraulic hybrid vehicle the working fluid can be applied to a pump under pressure. Regenerative braking may operate to supplement or replace operation of the conventional service brakes, in a fashion similar to an engine brake or retarder in the drive line on a conventional vehicle. The torque absorbed for regeneration supplements the braking torque requested by the driver by use of the brake pedal. Absent compensating brake pedal resistance, this results in the vehicle stopping faster for a given brake pedal input and biases the braking force to the drive axle(s).
In a full hybrid or electric vehicle, the vehicle's electric traction motor doubles as the electrical generator which can be coupled to be driven by the wheels. On a hydraulic hybrid vehicle a pump may be coupled to the driveline. Typically only some of the wheels are driven, and thus capable of being coupled to the electric traction motor/generator or hydraulic pump when it is operating in its generating/storage mode. Thus, on either type of vehicle, a portion of the braking torque will come from the service brakes mounted with non-driven wheels, though braking force is biased toward the drive axles as they receive both service brake torque and regeneration torque while the non-drive axle(s) receive only service brake torque. Consideration may be given the issue of anti-lock braking systems (ABS) which distribute braking force to maintain braking stability.
U.S. Pat. No. 6,454,365 describes a braking force control system for a vehicle incorporating hydraulic service brakes and regenerative braking for the vehicle's drive wheels. The '365 patent provides a braking controller which generates a target braking force for front and rear wheels of the vehicle. Initially the controller applies regenerative braking in attempting to meet the target braking force levels. If regenerative braking proves insufficient to meet braking target levels, friction service brake operation is added to any wheels not supplying the target level of braking torque.